The present invention relates to servo systems for track-following longitudinal tape movement in a longitudinal direction, and more particularly, to fast center calibration of a tape drive that is used with a flangeless tape path.
The function of a servo system for longitudinal tape, such as magnetic tape, is to move a head laterally in respect to the longitudinal tape to accurately follow the lateral movement of the tape, for example, during read/write operations of the head. If done accurately, the data tracks are written and read in straight lines along the longitudinal tape as the tape is moved in the longitudinal direction. With respect to magnetic tape, the data comprises parallel stripes in the longitudinal direction of the magnetic tape. Servo tracks are prerecorded in the magnetic tape parallel to, and offset from, the expected data stripes. Typically, the lateral movement of the magnetic tape is constrained by flanges present on tape guides at either side of the head, such that the servo system causes the head to follow the data stripes in the presence of disturbances mainly created from limited lateral motion of the tape, called LTM (Lateral Tape Motion) or tape excursions.
Servo systems often employ compound actuators to move the head laterally both for track following, and to shift from one servo track (or set of servo tracks) to another and to follow a different set of data stripes. A compound actuator, which comprises a coarse actuator and a fine actuator mounted on the coarse actuator, provides both a large working dynamic range and high bandwidth. The high bandwidth fine actuator typically has a limited range of travel to attain the high bandwidth, and, in the typical track following arrangement, with the fine actuator as the master and the coarse actuator as a slave to the movement of the fine actuator, if the fine actuator drifts to one side as the tape moves laterally, the coarse actuator follows (at a slower rate) the centerline of the movement of the fine actuator. This action is discussed in more detail in U.S. Pat. No. 6,587,303, issued Jul. 1, 2003, which is herein incorporated by reference.
The flanges of the tape guides, such as rollers, limit the lateral motion of the tape, but may tend to flex the tape and to introduce debris accumulation of the flanges that impact the lifetime of the tape and in addition create undesirable dynamic effects.
Flangeless tape guides, such as those used by IBM® LTO Generation 5 tape drives, IBM® 3592E07 tape drives and later, among others, tend to solve the problems of the flanged tape guides, but, without being constrained, the longitudinal tape tends to rapidly shift from one side of the tape guides to the other, and to run at one side of the guides for only a short period. Thus, in an attempt to follow the tape from one side to the other, the coarse actuator, in following the centerline of the movement of the fine actuator, is required to move from side to side as the tape rapidly shifts. This motion tends to wear and shorten the life of the coarse actuator, and is a use of power by the coarse actuator
In flangeless tape path designs, LTM or tape excursions may exceed a range of a fine actuator if the tape is not centered to the excursions as LTM occurs. This is because the fine actuator has a limited range of motion, and is designed such that it can scan the width of the tape. Therefore, if the head is positioned near a top or bottom of the longitudinal tape before fine actuation is performed, there is a chance that the fine actuator may not be able to drive the head to the other side of the longitudinal tape. In order to account for this deficiency, the drive must determine the midpoint of lateral tape excursions and place a coarse actuator system at this midpoint to allow the fine actuator to track-follow the longitudinal tape regardless of the excursions moving up or the excursions moving down. Finding the midpoint of the lateral tape excursion is not easy, as the midpoint of the lateral tape excursion is unique to each drive and the coarse and fine actuator systems have no absolute reference positions relative to the longitudinal tape path. Accordingly, the midpoint of the lateral tape excursion must be identified individually for each drive.
One method to find the midpoint of the lateral tape excursion when the tape moves between the top-most position and bottom-most position is fairly straight forward, and has been previously described in U.S. patent application Ser. No. 12/612,403, filed Nov. 4, 2009, which is herein incorporated by reference. However, some tapes behave in ways that make it difficult to find these extremes from which the midpoint of the lateral tape excursion may be calculated. Some tapes exhibit a behavior in which the tape is always or almost always in the top-most position in relation to the flangeless supply and take-up reels. Other tapes exhibit a behavior where the tape is always or almost always in the bottom-most position in relation to the flangeless supply and take-up reels. This makes calculating the midpoint of the lateral tape excursion for the tape drive impossible using existing methods, since LTM does not occur to an extent necessary to observe full lateral movement from which a midpoint may be calculated. Other tapes exhibit small excursions (also referred to as “runts”) away from one extreme but do not move completely to the other extreme. This behavior also makes it difficult to determine a proper midpoint of the lateral tape excursion or even sense the tape excursions.
Using typical centering methods, the tape position is approximated or determined by integrating the current flowing to the fine actuator, referred to as an integrator value. Another problem in determining the midpoint of the lateral tape excursion is that the current contains fine actuator track-following current, and thus integrated track-following values, that are a function of the reel run-outs as well as tape motion, which may typically include excursions.
Current tape centering methods require that both the top- and bottom-most positions be determined before a midpoint of the lateral tape excursion can be calculated. In order to determine the top- and bottom-most positions, often it is necessary to move the tape from beginning of tape (BOT) to end of tape (EOT), and then back to BOT with the anticipation that sufficient tape excursions will occur in both directions to be able to calculate the midpoint of the lateral tape excursion. If the top- or bottom-most positions are not properly detected, an incorrect midpoint of the lateral tape excursion may be calculated and the drive will not behave properly and may even cause a condition where the drive will no longer function if an inaccurate midpoint is calculated.